Ultrafiltration is a technique employed in the dairy industry to provide the selective separation, concentration and purification of protein components from a liquid dairy product such as raw milk. The technique comprises the steps of separating the liquid dairy product into a first ultrafiltrate or permeate liquid component by placing the dairy product in contact with a semi-permeable membrane. The first liquid component is the permeate which flows through the semi-permeable membrane. The second liquid component is the concentrate that does not flow through the semi-permeable membrane and which has a higher concentration than the first liquid component.
In practice, the first liquid component or permeate is collected and eventually disposed of on the farm or distributed as feed to livestock. The second liquid component or retentate continues to be separated until a milk concentrate with the desired enriched protein content is obtained. The milk product to be processed in contact with the semi-permeable membrane is placed under pressure and typically subjected to a turbulent flow so as to agitate the liquids adjacent the membrane and enable the obtaining of a higher content of solids in the retained liquid concentrate. Using on-farm ultrafiltration, dairy personnel and processors realize lower hauling costs for the reduced volumes of concentrate which must be transported to dairies, reuse of permeate as feed to livestock, reduction in refrigeration expenditures, and premium component pricing from enhanced value to the processor. In addition, ultrafiltration provides for increased cheese yields, a more consistent and predictable quality cheese and reduced rennet requirement.
In previously known milk processing systems, ultrafiltration has been typically performed in closed loop or recirculatory designs and at high concentration levels and/or at high temperatures (above 120.degree. F.) selected to provide increased yields and maximum membrane flux. The application of heat and pasteurization to control microbial growth has also been advocated with the objective of killing a portion of the bacteria present in milk or the retentate. Unfortunately, several species of bacteria continue to flourish and grow at such high temperatures and in such closed environments. In addition, the ultrafiltration concentrate that has been obtained by processing pasteurized milk is still required to be pasteurized at the destination plant to which it is shipped before it can be used in that destination if the milk and milk products are to retain a grade A certification. It has been discovered by cheese makers, that as milk is subjected to multiple heat treatment, changes may occur in the milk constituents that can result in the production of defective cheese.
Since all macromolecules and microbes are retained in ultrafiltration, there has been heightened concern about the presence of undesirable bacteria, toxins and enzymes. Therefore, it is a general aim of the present invention to provide an ultrafiltration system in which the growth of bacteria is minimal, heat damage to proteins and cheese defects is reduced and the advantages of low level concentration are maximized. It is also desirable to provide a method of processing milk to obtain an acceptable concentrate product which is not pasteurized or homogenized so as to maintain the highest quality of end product produced with the concentrate. It is further desirable to process high quality milk produced on the farm into membrane concentrated lactose reduced raw milk which will meet and exceed grade A standards for whole milk as regulated by the Food and Drug Administration (FDA) and the United States Department of Agriculture (USDA). It is within the purview of this invention to consider an alternative scheme to the prior art closed loop or recirculatory systems in order to avoid the problem of bacteria buildup in the ultrafiltration system during concentration.
It is also important in any membrane process to clean the membranes back to their original permeability so as to maintain maximum performance with respect to permeate flux. Both organic foulants, which are primarily proteins, and mineral foulants abound in milk and can significantly alter membrane permeability. The operating parameters of velocity and pressure must be chosen to minimize fouling and ensure membrane recovery. Likewise, consideration should also be given to minimizing polarization or saturated layers which build up next to the membrane if fluid flows through the membrane faster than the retentate can diffuse into the bulk liquid.